Historical zoogeography and current status of two species of hybridizing ground squirrels (Ictidomys parvidens and I. tridecemlineatus) on the southern plains of Texas

TEXAS TECH UNIVERSITY

Natural Science Research Laboratory

Occasional Papers

Museum of Texas Tech University

Number 314 6 December 2012

Historical Zoogeography and Current Status of Two Species of
Hybridizing Ground Squirrels (Ictidomys parvidens and I. tridecemlineatus)

on the Southern Plains of Texas

Frederick B. Stangl, Jr., Amanda L. Snook, Laura A. Harmon, and Dana R. Mills

Abstract

Zimmerman and Cothran (1976) and a series of their follow-up studies documented hy¬
bridization between the Rio Grande (. Ictidomys parvidens) and thirteen-lined ground squirrels
(7. tridecemlineatus). More than 20 years after their work, we applied a series of morphological
characters obtained from captive-bom Fj animals and field-collected specimens from across north
Texas. Hybridization remains a localized phenomenon, with hybrid individuals comprising the
majority of resident animals in Baylor County, Texas. Ranges have changed little in the interven¬
ing decades, but our collecting efforts have generated marginal county records for 7. parvidens
from Jones, Shackleford, Stonewall, and Throckmorton counties. Extirpation of two populations
of I. tridecemlineatus sampled by Cothran (1982, 1983) were documented in Wilbarger and
Motley counties — the latter site now occupied by the Rio Grande squirrel. Historical accounts
suggest that hybridization between the two taxa likely was initiated in the mid-1900s by range
expansion of 7 parvidens , a mid-Holocene immigrant into Texas from Mexico.

Key words: ground squirrels, hybridization, Ictidomys parvidens , Ictidomys tridecemlin¬
eatus , Southern Plains, sibling species, zoogeography

Introduction

The recent revision of Holarctic ground squir¬
rels by Helgen et al. (2009) recognizes eight genera,
while restricting the use of Spermophilus to Old
World taxa. As presently defined, Ictidomys is now
restricted to three species: the thirteen-lined ground
squirrel (7. tridecemlineatus), and each of the two
previously recognized subspecies of Mexican ground
squirrel (7. mexicanus [Mexican ground squirrel] of

central Mexico, and the newly elevated 7. parvidens
[Rio Grande ground squirrel] of Texas, New Mexico,
and northeastern Mexico). Two of these species, 7.
tridecemlineatus and 7. parvidens , maintain a nearly
parapatric distribution in Texas, with the exception of
an area of broad apparent sympatry from northcentral
Texas into eastern New Mexico.

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Occasional Papers, Museum of Texas Tech University

The point of departure for this zoogeographic
discussion of Ictidomys from the Southern Plains is
the late Pleistocene. Most regional fossil and subfossil
faunas from the study area are from alluvial sediments
of the Kansan Province in Texas and Oklahoma and
the limestone cave silts of the Balconian and Chihua-
huan provinces in Texas and New Mexico. Although
ground squirrel dentition is usually distinctive at the
generic level sensu Helgen et al. (2009; e.g., Bryant
1945; Goodwin 2009), identifiable ground squirrel
material is often scarce, and such remains are com¬
monly edentulous jaws or isolated teeth that may be
sufficiently worn or weathered as to obliterate distinc¬
tive dental characters. Further, there is no single di¬
agnostic character distinguishing Ictidomys parvidens
from/, tridecemlineatus. Size alone is of limited utility
(Cothran 1982, 1983), and the potential for temporal
size variation has been demonstrated for sciurids and
a variety of other taxa (Martin and Barnosky 1993,
and citations therein). A final confounding factor is
the purported regional Pleistocene presence of at least
two other ground squirrels with dentition comparable in
size to I. parvidens — Richardson’s ground squirrel of
the Urocitellus richardsonii-elegans complex, and the
larger Franklin’s ground squirrel with proportionally
smaller teeth, Poliocitellus franklinii. These sciurids
occur no closer today than the High Plains of Colorado
and Kansas, respectively, but fossil material attributed
to these sciurids are recorded from the mesic Pleisto¬
cene grasslands and savannas south into Oklahoma
(Graham et al. 1987; Smith and Cifelli 2000), Texas
(Graham et al. 1987; Dalquest and Schultz 1992) and
New Mexico (Harris 1989).

The post-Pleistocene shift towards a warmer
and more arid environment was a complex, gradual,
and regionally variable phenomenon for the Southern
Plains states of Texas, Oklahoma, and New Mexico
(e.g., Semken 1983; Wright 1983; Davis 1987; Graham
et al. 1987; Harris 1990; Gehlbach 1991). The mid-
Holocene shift (3-8,000 YBP) seems especially critical
to the regional displacement of resident flora and fauna
by incursions of both xeric-adapted western species
and subtropical species from the south that continue to
this day. A series of woodrat midden studies (e.g., Van
Devender et al. 1987a, b; Elias and Van Devender 1990)
have chronicled the Trans-Pecos Texas transition of the
prevailing early Holocene pinyon-oak-juniper wood¬

lands and savanna to mid-Holocene desert grasslands,
and the nearly complete emplacement by about 4,000
YBP of the modern desert flora and fauna.

Our knowledge of the earlier historical distribu¬
tions of the two Ictidomys is limited, but given a predi¬
lection for the altered habitats around early settlements,
military installations, and individual homesteads, I.
parvidens and I. tridecemlineatus were conspicuous
residents to early European immigrants. The fanciful
vernacular terms applied by early settlers to I tride¬
cemlineatus are evidence for their degree of familiarity
with the distinctive pattern (e.g., “leopard spermophile”
of Goodrich and Winchell 1894; “federation squirrel”
of Baird 1857—the latter a reference to the alternating
rows of spots and stripes resembling the American flag).
Common names for/, parvidens usually referred to the
presumed northern limits of the species’ geographic
distribution in Texas (e.g., Mexican ground squirrel of
Baird 1857; Rio Grande squirrel of Bailey 1905).

Congruence of molecular (Herron et al. 2004) and
dental (Goodwin 2009) studies suggest that/ parvidens
(Rio Grande ground squirrel, sensu Helgen et al. 2009)
is related more closely to I. tridecemlineatus than to
the previously conspecific Z mexicanus. The largely
exclusive ranges ofZ parvidens and/ tridecemlineatus
encompass much of the Great Plains and converge in
the Southern Plains of north Texas and eastern New
Mexico (Hall 1981; Schmidly 2004). The thirteen-lined
ground squirrel is largely a prairie animal, ranging from
Texas to southern provinces of Canada, although along
the western margins of its range, isolated populations
occupy high montane parklands. The Rio Grande
ground squirrel occurs from the semiarid scrub and
desert grasslands of northeastern Mexico to the mes-
quite savannas of northern Texas.

The two taxa have accumulated little differentia¬
tion since their late Pleistocene divergence at the mor¬
phological (Bryant 1945; Black 1972; Cothran 1983;
Goodwin 2009), chromosomal (Nadler and Hughes
1966; Zimmerman and Cothran 1976; Cothran and
Honeycutt 1984), allozymic (Cothran et al. 1977; Co¬
thran 1983) or molecular (Harrison et al. 2003; Herron
et al. 2004) levels. Nevertheless, the two are superfi¬
cially distinctive. The thirteen-lined ground squirrel is
the smaller sciurid, seldom weighing in excess of 150

Stangl, Jr., et al.—Ground Squirrels ( Ictidomys ) on Texas Southern Plains

3

g. The dark dorsal pelage is marked by rows of white
spots against a dark dorsum, with the spots of alternate
rows confluent to form solid stripes. The Rio Grande
ground squirrel is a larger and rangier animal that can
exceed 300 g, and sports both a proportionally longer
tail and series of distinct spots arranged in rows against
a comparatively pale tan-to-gray dorsum.

Typical of ground squirrels, the two taxa are
often conspicuous residents near human habitations,
with their diurnal habits, preference for open terrain,
and their characteristic alarm trills. While each species
exhibits rather broad ecological tolerances, both prefer
sparsely vegetated or short-grass settings. Densest
populations today usually occur in artificially enhanced
and manicured habitats (e.g., golf courses, parks, school
campuses). Especially favored are cemeteries, which
are often established at the edges of communities and
smaller towns, experience light and sporadic human
traffic, and provide favorable burrowing conditions
related to the loosened and friable fill of grave sites—a
feature especially useful in areas of compact or rocky
soils. Animals living under more natural conditions
(e.g., rangelands and pastures) tend to be sparsely dis¬
tributed and more secretive, although local exceptions
sometimes occur where moderate to heavy grazing re¬
gimes are practiced. Roadways and railroad rights-of-
way provide seemingly logical dispersal routes across
and through otherwise unsuitable terrain (e.g., dense
brush, areas of monoculture), at least where vegetation
is sparse or mowed regularly.

Within the relatively broad band of mapped sym-
patry across southeastern New Mexico and north Texas
(Figs, la, b),/. tridecemlineatus is the prevalent species
across all but the southern periphery of the High Plains
(Choate 1997), where/. parvidens is common. Howev¬
er, microallopatry seems a more accurate term here, for
nowhere have the two taxa been taken simultaneously
from the same locality. Contact has certainly occurred
in places, for there are well documented instances of
hybrid individuals—in each instance, coexisting with
parental I. parvidens —within this area of apparent
sympatry (Zimmerman and Cothran 1976; Cothran
1982, 1983; Cothran and Honeycutt 1984).

This study assesses the zoogeographic history and
current status of I. parvidens and I. tridecemlineatus
by comparing the results of an extensive systematic
collecting effort with existing collection records and
earlier literature. Documenting the current distributions
of these species permits comparisons for the tracking
of any future range shifts, as have been noted for other
components of their respective faunas over the past
several decades. During just the past century, mammals
sharing subtropical affinities with/, parvidens (notably
Dasypus novemcinctus, Taulman and Robbins 1996;
Baiomys taylori, Choate et al. 1990) have extended their
ranges for hundreds of miles northward onto the Rolling
Plains of Texas and beyond. Ictidomys tridecemlinea¬
tus is largely a species of more mesic prairie environs,
and Frey (1992) provides examples of boreal species
simultaneously extending their ranges southward into
the Southern Plains (e.g., Microtus pennsylvanicus,
Zapus hudsonius, Mustela nivalis). At least one North¬
ern Plains mammal, Microtus ochrogaster, is actively
reclaiming parts of its southern Pleistocene range in the
Texas Panhandle (Choate and Killebrew 1991; Stangl
et al. 2004; Poole and Matlack 2007).

A necessary corollary to documenting the pres¬
ent status of the two Ictidomys is the extent of detect¬
able hybridization. While lacking fixed karyotypic
or allozymic markers to distinguish the two species,
Cothran (1982, 1983) was at least able to demonstrate
the morphometrically intermediate distinctness of
most of his “purported” hybrids. We provide a simple
and reliable field method for distinguishing presump¬
tive hybrid individuals from members of the parental
species. While molecular analyses will provide a
definitive determination of hybrid origin for any given
specimen, identification of the distribution and numbers
of purported hybrids by superficial or morphological
means can certainly provide a coarse measure of the
extent of hybridization and introgression between the
two species (e.g., Homyack et al. 2008 for Lynx rufus
x L. canadensis ) and the geographic framework for the
focus and design of subsequent genetic studies.

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Occasional Papers, Museum of Texas Tech University

c) - vegetation regions

a-desert scrub savanna
b-plains grassland
c-mesquite savanna
d-oak forest & prairies
e-oak/hickory/pine forest
f-blackland prairie
g-oak/hickory forest
h-coastal prairie
i-mesquite/chaparral savanna
j-juniper/oak/mesquite savanna

d) - biotic provinces

Figure 1. Current known distribution of Ictidomysparvidens (a) and I. tridecemlineatus (b) in Texas and proximal
regions of adjoining states, superimposed over known range in early 1900s (hatched, from Bailey 1905); (c)
regional vegetation map (modified from Kuchler 1964); and (d) biotic provinces (modified from Blair 1950).
Closed circles from within the 31-county study area of north-central Texas represent counties where specimens
are documented from literature records and from this study.

Stangl, Jr., et al.—Ground Squirrels ( Ictidomys ) on Texas Southern Plains

5

Materials and Methods

The study area was designed to encompass the
interface and overlap between northern populations
of Ictidomys parvidens and the southern extent of
the range of I. tridecemlineatus across the Southern
Plains of north-central Texas. Cemeteries of mapped
communities from 31 contiguous counties across the
region were canvassed during the late spring to early
fall months of 2004-2007 for ground squirrels.

Description of study area. —The terrain of the
Southern Plains (Kansan and Balconian) and adjoining
regions, combined with prevailing weather patterns,
produce modern vegetation regions (Fig. lc; from
Kuchler 1964) coincidental with the distinct plant-
animal associations recognized by Blair (1950) as
biotic provinces (Fig. Id). The effects of topography
and edaphic considerations are also obvious: the Rocky
Mountain flanks and foothills of the Navahonian that
extend into the Chihuahuan; the Kansan, comprised
of the semiarid and level High Plains (ca. 975+ m)
of the west and the gently eroded Rolling Plains to
the east of the Caprock Escarpment; the low and rug¬
ged terrain of the Edwards Plateau, comprising the
Balconian; the desert mountain-and-basin system of
the Chihuahuan; the subtropical Tamaulipan; and the
Austroriparian woodlands that merge westward into the
ecotonal Texan, with its alternating bands of prairies
and forests.

Some physical boundaries are more sharply de¬
marcated than others (e.g., escarpments defining both
the Edwards Plateau and the High Plains of the western
Kansan Province). As the post-Pleistocene composition
of floral and faunal associations changed in response
to climatic shifts, the relatively static nature of such
physical features as topography and soils appear to have
continued to define the margins of what are essentially
in situ evolving communities, even as their composi¬
tions changed. As such, we find it useful to retain the
application of these biotic provinces dating back to the
latest Pleistocene.

Two important compendia (Graham et al. 1987;
Wright 1983) and a series of other studies (e.g., Hibbard
and Taylor 1960; Van Devender et al. 1987 a, b; Elias
and Van Devender 1990; Harris 1987,1990; Gehlbach
1991; Dalquest and Schultz, 1992; Hafner 1993; Stangl
et al. 1994; Smith and Cifelli 2000) have permitted our

following overview of the regional paleoecology and
Ictidomys biogeography since our point of departure—
the latest Pleistocene, when deciduous woodlands
occupied the Texan and Austroriparian provinces.
Mixed to tail-grass prairies dominated the High Plains
of the Kansan, and along the northwestern margins of
the Texan. Steppe conditions prevailed across lower
elevations to the south and west, with sage dominating
much of the Chihuahuan and Navahonian provinces,
and chaparral throughout the Tamaulipan. Savannas
of oak, juniper, and pinyon pine cloaked the Edwards
Plateau and extended into more rugged parts of the
southern Rolling Plains.

Sampling methods. —Our past field experiences
have demonstrated that cemeteries would be ideal
collecting localities for ground squirrels. We visited
such sites for most mapped communities on a 2000
Texas roadmap. Cemeteries lacking evidence of
ground squirrels and judged unsuitable for a variety
of reasons (e.g., too small, overgrown with vegetation,
surrounded by extensive areas of unsuitable habitat)
were not revisited. Other cemeteries deemed suitable
habitat were visited at least three times, and searched
on foot for sign of squirrel activity, before concluding
that no ground squirrels resided locally. Such instances
occasioned the need to search for specimens at nearby
fair grounds, school campuses, or golf courses. Three
cemeteries with excessively sandy soils (Estelline of
Hall County, Knox City of Knox County, and Roar¬
ing Springs of Motley County) supported only small
populations of the spotted squirrel, Xerospermophilus
spilosoma.

Animals were captured by hand after flooding of
burrows and transported to the laboratory for process¬
ing. Sample sizes were determined by a subjective
assessment of resident population size, with a minimum
of two individuals whenever possible. Geographic
samples of > 6 represented sites collected over a period
or two or more years. Each specimen was measured
by the senior author and prepared as a study skin with
skeleton for deposit in the Collection of Recent Mam¬
mals of Midwestern State University (MWSU). Tissues
(liver, heart, kidney, muscle) were retained for deposit
in the Genetic Resources Collection at the Museum of
Texas Tech University.

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Occasional Papers, Museum of Texas Tech University

Roadsides were visually surveyed during our
travels over the entire three-year collecting effort and
during the course of unrelated field studies. Particular
vigilance was maintained in areas separating known
populations of both species.

Determination of hybrid individuals. —A refer¬
ence litter of three Fj hybrid animals (MWSU 22564-
22566) was obtained from the laboratory cross of a
female I. parvidens with a male I. tridecemlineatus.
Each possessed the paternal spot-and-stripe pattern
against a background color only slightly darker than
the maternal condition. Compared with reference series
of comparable-aged (75 days-of-age) specimens of I.
parvidens and 7. tridecemlineatus in the MWSU collec¬
tion, hybrids were intermediate in size for external body
dimensions (Fig. 2). Attempts to produce offspring via
reciprocal crosses failed.

Combined with geographic locality, pelage fea¬
tures, size, and ambulatory gait, we deemed it possible
to identify many hybrids in the field. All field-caught
specimens of suspected hybrid origin (e.g., backcrosses,

&s, F 0 s) retained the alternating spot-and-stripe pat¬
tern, although background color intensity sometimes
approached that of I. tridecemlineatus , and body di¬
mensions sometimes approached the lower limits of
I. parvidens. Most of these purported hybrids can be
recognized in the field prior to capture and handling—
providing the general impression of a large, rangy I.
tridecemlineatus with the distinctive tail-held-high
gallop of a Rio Grande ground squirrel. Examination
of study skins from Cothran’s Baylor County hybrid
sample agree in appearance and dimensions with pur¬
ported hybrids reported from our investigation.

Confirmation of each presumed hybrid deter¬
mination was obtained by greatest length of the hind
foot, the most highly repeatable of the four standard
external measurements, and the one which can even be
obtained reliably from a live specimen in hand. The
hind foot length achieves adult dimensions by 75-days-
of-age (Stangl et al. 1995), permitting application of
this method on all but the younger juveniles (/. tride¬
cemlineatus < 90 g, I. parvidens < 130 g, purported
hybrids < 100 g) taken in this study. A measure of the

Figure 2. Study skins of Ictidomysparvidens from Stonewall County, Texas (top; MWSU 22373) and I. tridecemlineatus
from Wichita County, Texas (bottom; MWSU 22303), bracketing field-taken hybrid ground squirrel from Baylor
County, Texas (center; MWSU 22440). Hybrid animal closely resembles captive-born FjS in size, pelage features,
and body dimensions.

Stangl, Jr., et al.—Ground Squirrels (Ictidomys) on Texas Southern Plains

7

repeatability of this character is the close agreement of
our hind foot measurements (Table 1) with those for
adults of the two species reported by Cothran (1983:
mean of 34.3 ± 1.7 SD for/. tridecemlineatus', mean of
40.6 ± 2.6 SD for I. parvidens). Reliability of hind foot

measurements to distinguish parentals from individuals
of hybrid origin among field-caught specimens was
tested with a one-way analysis of variance and Dun¬
can’s multiple means tests, using the NCSS statistical
package (Hintze 1990).

Table 1. Hindfoot measurements of adult Ictidomys parvidens (weights > 130 g), adult I. tridecemlineatus (weights >
90 g), and purported hybrid individuals initially identified on the basis of morphological characters from 19 counties

in north-central Texas. Descriptive statistics are: sample size (n), mean, standard deviation (SD), range (minimum-
maximum), confidence intervals (C.I.), and coefficients of variation (CV). Analysis of variance: P < 0.0001; Duncan’s
Multiple Means Test: each of three subsets (parental species and hybrids) significantly different from one another at
P < 0.05 level.

Hind foot measurement (mm)

County (n)

mean ± SD

range 95% C.I. CV

Ictidomys parvidens

Baylor (1)

39.0

-

Cottle (4)

40.8

40-41

Dickens (6)

41.7

40-44

Fisher (2)

40.0

39-41

Garza (4)

41.0

40-42

Haskell (2)

41.5

41-42

Jones(13)

40.9

40-45

Kent (2)

42.0

41-43

Knox (7)

40.1

38-42

Motley (4)

39.8

37-42

Shackleford (5)

41.2

39-43

Stonewall (6)

42.5

41-43

Throckmorton (9)

40.4

39-44

Total (65)

40.9 ± 1.5

37-45 40.5-41.2 3.6

Ictidomys tridecemlineatus

Archer (2)

35.5

35-36

Floyd (2)

34.5

34-35

Hardeman (4)

34.3

33-35

Wichita (19)

33.3

32-35

Wilbarger (3)

34.6

34-36

Total (30)

33.5 ± 1.2

32-36 33.0-33.9 3.7

Ictidomys hybrids

Baylor (9)

38.0

36-40

Haskell (1)

38.0

Knox(l)

38.0

-

Throckmorton (3)

37.6

36-40

Laboratory FjS (3)

35.3

35-36

Total (17)

38.0 ± 1.9

35-40 37.0-39.0 4.9

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Occasional Papers, Museum of Texas Tech University

Results

Most of the westernmost counties of our study
area are dominated by large ranches or extensive
monoculture (usually cotton) and comparatively few
communities, thereby providing fewer collecting op¬
portunities (i.e., fewer cemeteries or golf courses).
Ground squirrels were collected from 21 of the 31
Texas counties surveyed (Appendix). No single county
during the time of our survey provided both Ictidomys
parvidens and I. tridecemlineatus (Fig. 3). The Rio
Grande ground squirrel was taken from 14 counties,
and four of these (Jones, Shackleford, Stonewall, and
Throckmorton) are new county records. The thirteen-
lined ground squirrel was taken from six counties. Two
regions appeared to be uninhabited by either species of

Ictidomys : the northwesternmost corner (Briscoe, Chil¬
dress, Hall counties), where rugged terrain or extensive
sandy soils are prevalent; and the easternmost region
(Clay, Jack, Montague, Palo Pinto, Stephens, Young
counties) of the Cross Timbers. Specimens of the Rio
Grande ground squirrel were obtained incidental to
unrelated field studies from King (MWSU 22571) and
Knox (MWSU 22570) counties. Visual sightings from
along roadways during our travels were rare: single
presumed I. parvidens were observed, but not collected,
from each of Dickens, King, and Knox counties.

General distributions of the two taxa have not
changed dramatically since Cothran’s (1983) study,

Figure 3. Map of 31-county study area and collecting localities in north-central Texas from 2004 to 2007.
Results are: closed circles, Ictidomys parvidens ; circles with enclosed “t”, I. tridecemlineatus ; open circles,
absence of Ictidomys.

Stangl, Jr., et al.—Ground Squirrels ( Ictidomys ) on Texas Southern Plains

9

although specific findings worthy of note included:
1) extirpation of I. tridecemlineatus from Vernon in
Wilbarger County since Cothran (1982); 2) extirpation
of I. tridecemlineatus from Matador in Motley County
since Cothran (1982), with subsequent replacement by
I. parvidens', 3) occurrence of/, tridecemlineatus near
Lockett of Wilbarger County, where Dalquest (1968)
reported a specimen of I. parvidens', and 4) determined
presence only of/, parvidens in Stonewall County, from
where both species were taken earlier in close proximity
(Ruhl and Stangl 1997).

Significant variation was observed for hind foot
measurements (Table 1) of I. parvidens, I. tridecem¬
lineatus , and individuals of hybrid origin (one-way
ANOVA; P < 0.0001), with each of the three classes

varying significantly from one another (Duncan’s
multiple means test; P < 0.05). There was no overlap
between the two parental species. Reflecting the ar¬
ray of recombinant possibilities, measurements from
the hybrid sample demonstrated the greatest range of
variation (CV = 4.9), were generally intermediate in
size, and exhibited minimal overlap with the smaller
I. tridecemlineatus.

Purported hybrids, as determined by our criteria,
were taken from localities in four contiguous counties
that also supported I. parvidens. The Baylor County
sample was comprised almost exclusively of hybrid
animals (15 of 16), while purported hybrids comprised
the minority in Haskell (1 of 4), Knox (2 of 5), and
Throckmorton (3 of 12) counties.

Discussion

We propose that the thirteen-lined ground squir¬
rel was the only regional Ictidomys during the late
Pleistocene, occurring essentially throughout the
higher plains of the western Kansan Province and
sporadically over the contiguous Navahonian region
of our study area. Extension into the Texan Province
as far south as the Gulf Coast would have been a late
Holocene (and perhaps very recent) phenomenon, as
woodlands retreated eastward and human settlement
further cleared the land. Ictidomys tridecemlineatus has
a long and well-documented Pleistocene record across
the Central Plains as far north as Wyoming and as far
east as Pennsylvania (e.g., Kurten and Anderson 1980;
Graham et al. 1987). The species has continually oc¬
cupied essentially the western half of its modern South
Plains distribution in Texas and Oklahoma (Johnson
1986; Dalquest and Schultz 1992; Smith and Cifelli
2000). More southerly extralimital populations in the
vicinities of Carlsbad, New Mexico (Harris 1989) and
El Paso, Texas (Van Devender et al. 1987a) vanished
with the advent of regional mid-Holocene desertifica¬
tion, perhaps coincidentally with colonizations of the
mountain parklands of New Mexico and Arizona at
elevations as high as 2600 m (Bailey 1931). Eastern
populations that extend as far south as the Houston
vicinity on the Texas Gulf Coast were first noted by
Bailey (1905) and probably originated via immigration
along the emerging parklands and prairie corridors of
the modern Texan Province as the eastern woodlands

retreated eastward. The extent to which settlement
and land clearing practices facilitated this southern
dispersal is uncertain.

Alternatively, the fossil record for the Rio
Grande squirrel is problematic. There are no Pleis¬
tocene examples from Mexico for either I. parvidens
or I. mexicanus (Arroyo-Cabrales and Polaco 2003),
and few, if any, reliable records exist for I. parvidens
from any of a series of Pleistocene faunas within the
present range of this sciurid (Graham 1987, Van De¬
vender 1987a, and Stangl et al. 1994 for west Texas;
Harris 1987,1989 for southeastern New Mexico). The
original basis for a presumed Pleistocene presence of
I. parvidens in north-central Texas is an isolated tooth
initially identified as either Spermophilus mexicanus
(= I. parvidens) or U. richardsonii (Dalquest 1965),
but later assigned after reexamination to U. elegans
(Dalquest and Schultz 1992). The other oft-cited
Pleistocene record for I. parvidens is an isolated tooth
of earliest Holocene age (ca. 9,000 YBP; Dalquest et
al. 1969) from Schultz Cave on the southwestern Ed¬
wards Plateau, which Dalquest and Schultz (1992) later
noted was indistinguishable from either I. parvidens
or U. richardsonii. Since these earlier reports, both I.
parvidens and I. tridecemlineatus were recorded with
U. richardsonii/elegans from the same Pleistocene
and early Holocene sediments of the southern Texas
Panhandle that also contained such mesic grassland/

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Occasional Papers, Museum of Texas Tech University

savanna taxa as Microtus and Synaptomys (Johnson
1986). Against both historical and ecological contexts,
these latter Panhandle materials attributed to the Rio
Grande ground squirrel appear incongruous and war¬
rant reexamination.

The mid-Holocene warm cycle seems to coincide
with the invasion from the Tamaulipan Province of
Mexico by 7. parvidens and its advancement westward
along the Rio Grande and Pecos River valleys towards
the western limits of its present range. The species first
appears in a series of southwestern Balconian cave
sediments and rock shelters dating 3-6,000 YBP (for
summary, see Graham 1987). These sites are proximal
to where Chihuahuan, Balconian, and Tamaulipan prov¬
inces converge and were inhabited by Paleo-Indians,
suggesting the animals were brought into the sites as
food items.

Baird (1857) recorded 7 parvidens from along
and on either side of the Rio Grande and north along
the Gulf Coast to Corpus Christi, suggesting to him that
the northern limits of this “Rio Grande squirrel” were
largely coincidental with the river. His misstatement
that the species occurred as far west along the river
as Fort Bliss (vicinity of El Paso, a locality more than
200 km west of any known record for the species in
Texas or New Mexico) was doubtless in reference to
his reported specimen of/, tridecemlineatus from a now
extirpated population from Fort Thorn of that vicinity.
The species today occurs along the Rio Grande no
farther than the eastern boundary of Big Bend National
Park of Brewster County (Borrell and Bryant 1942;
Schmidly 1977) and no farther west in the Chihuahuan
Province of Texas than the Davis Mountains of Jeff
Davis County (Blair 1940) and Apache Mountains of
Culberson County (Dalquest and Stangl 1986; Stangl
etal. 1994).

One of the military expeditions relied upon by
Baird (1857) for specimens was led by Capt. Randolph
Marcy, a noted naturalist. He made no mention of any
ground squirrels in his detailed notes (Marcy 1856) as
he traversed a horizontal figure-8 trail (as mapped by
Neighbors 1954) through the central two-thirds of our
study area in the Rolling Plains of north-central Texas.
The slow and leisurely travel afforded by horseback and
wagon from mid-July to mid-August 1854 by Marcy
was certainly conducive to observations of the diurnal

fauna, and it is difficult to believe that detection by
sighting of animals or the sound of the squirrels’ alarm
calls would have escaped his notice.

Modern (post-1900) Zoogeography of
Ictidomys.—Vernon Bailey’s (1905) treatment of Texas
mammals reflected extensive and systematic collecting
across most of the state. Excepting the eastern and
western extremities of Texas, his described range for
7. parvidens (extrapolated in Fig. la from his account)
and figured range of 7. tridecemlineatus (incorporated
into Fig. lb) suggested that one or the other species
occurred everywhere in our study area of the South
Plains but the northern mesquite savanna—the region
traversed by Marcy (1856). These distributions pro¬
posed by Bailey (1905) were little altered in portrayals
of the species’ ranges by Howell (1938), Taylor and
Davis (1947), and as recently as Davis (1974). Ab¬
sence of either species of Ictidomys from the northern
mesquite savanna of the Rolling Plains therefore does
not appear to be artifactual. It was not until the mid-
1900s (Blair 1954; Dalquest 1968) that the appreciable
expansion of the two species across north-central Texas
was first documented—7 parvidens towards the north
and east, and progression towards the conjoining of
eastern and western populations of 7. tridecemlineatus
in northcentral Texas. We have yet to find any super¬
ficial evidence in Texas or southwestern Oklahoma
for morphological intergradation in our study area
between the pale western populations (7. t. arenicola )
and their darker counterparts (7. t. texensis ) to the east
(Blair 1939; Dalquest et al. 1990; Stangl et al. 1992).
Much of the rugged terrain proximal to the Caprock
Escarpment, and extensively sandy areas in parts of the
eastern Texas Panhandle, clearly prevents colonization
by either species of Ictidomys.

We judge that contact of the ranges and sub¬
sequent sympatry of the two species was achieved
sometime in the mid-1900s, and that the sporadic
distribution of populations, and perhaps some degree
of localized habitat segregation, continue to ensure
that hybridization remains a localized phenomenon
within this zone of sympatry. Perhaps inspired by a
broad zone of mapped sympatry between 7. parvidens
and 7. tridecemlineatus (Hall 1981), Earl Zimmerman,
Gus Cothran, and colleagues (Zimmerman and Co¬
thran 1976; Cothran et al. 1977; Cothran 1982, 1983;
Cothran and Honeycutt 1984) proceeded to define the

Stangl, Jr., et al.—Ground Squirrels ( Ictidomys ) on Texas Southern Plains

11

nature and extent of hybridization between the two taxa
at the morphological, genetic, and karyotypic levels.
Among their findings are that hybridization is a local¬
ized phenomenon, that the two species did not coexist
at any site, and that hybrid individuals were noted only
among populations of I. parvidens.

Range expansion of I. parvidens to the north and
east across the mesquite savanna of Texas from at least
late 1960s to early 1970s has been dramatic, but more
than two decades have elapsed since Cothran’s (1983,
1984) efforts and this study, and little has changed.
Hybridization remains a localized phenomenon in
our study area, and mostly centered in Baylor County.
Further encroachment westward into the Chihuahuan
Province seems unlikely, given the markedly arid
and rugged desert terrain in Texas and New Mexico.
County records for I. parvidens reported herein (Jones,
Shackleford, Stonewall, and Throckmorton counties of
Texas) might represent further expansion of the species,
although we recognize that this region has historically
been neglected by collectors. The regional status of
the Rio Grande squirrel is presently secure.

Schmidly (2004) first called attention to declining
numbers of the thirteen-lined ground squirrel in north
Texas, attributing this trend to habitat degradation. This

would not seem to apply to such artificial habitat afford¬
ed by cemeteries, where we noted the extirpation of two
substantial populations of I. tridecemlineatus sampled
earlier by Cothran. F irst is Vernon of Wilbarger County,
which is presently unoccupied by any ground squirrels,
although we collected I. tridecemlineatus from Lockett,
less than 10 km to the south—a locality from where
Dalquest (1968) once reported a specimen of I. parv¬
idens. Second is Matador of Motley County, which is
presently occupied by 7. parvidens. We judge this latter
case an example of opportunistic colonization by the
Rio Grande squirrel following a complete extirpation
of I. tridecemlineatus , as there is no morphological
evidence of any residual hybridization that might be
expected following active displacement of one species
by the other. We cannot ascertain if these extirpations
are natural phenomena (e.g., epizootic event) or if
they are even related. However, control efforts on the
parts of groundskeepers are suspect, and such popula¬
tions are certainly vulnerable to control methods. At
least two golf courses within our study area actively
control ground squirrels on their grounds, although
most attitudes we encountered in Texas ranged from
protective to indifference. Nevertheless, we concur
with Schmidly’s (2004) concern about the conservation
status of I. tridecemlineatus.

Summary and Conclusions

Our interpretations of the literature and findings
reported herein suggest that western populations of
Ictidomys tridecemlineatus arenicola have likely oc¬
cupied the Navahonian and western Kansas provinces
since the late Pleistocene. The mid-Holocene shift
towards a warmer and more arid environment opened
an avenue to the Guff Coast via the Texan Province for
I. t. texensis. If further facilitated by human habitat
modification (e.g., agricultural practices, transportation
routes), then arrival of this eastern subspecies in the
vicinity of Houston, Texas, may even be an historical
event of the late 1800s or earliest 1900s. Morphologi¬
cal intergradation between the two subspecific taxa in
the Rolling Plains of the eastern Kansan has yet to be
demonstrated either in Texas (Dalquest et al. 1990;
Schmidly 2004) or in Oklahoma (Stangl et al. 1992).

The same Holocene climate shift that permitted
access of I. tridecemlineatus to the Guff Coast may
also be responsible for dispersal of/, parvidens beyond
the Rio Grande and into the southern Rolling Plains.
Subsequent range expansion of the Rio Grande squirrel
to the north and east may also have been facilitated by
human activity, and the contact and subsequent hybrid¬
ization with eastern populations of I. tridecemlineatus
may have occurred no earlier than the mid-1900s, as
first postulated by Cothran (1982).

The range of 7. tridecemlineatus appears to be
stable at present, although extirpation of at least two
populations in our study area suggest that Schmidly’s
(2004) call to monitor the species seems warranted.
Conversely, our collecting records suggest that the Rio

12

Occasional Papers, Museum of Texas Tech University

Grande squirrel has experienced some territorial gains
since the late 1900s (e.g., northernmost record for the
species in Motley County, new county records for five
north-central Texas counties).

Hybridization between the two Ictidomys is
ongoing, and hybrid individuals presently constitute
almost the entire population in sampled areas of Baylor
County, Texas. Lesser degrees of hybridization were
noted in contiguous counties of Haskell, Knox, and
Throckmorton. However, we note that the observed
range in variation for hybrid individuals of determin¬
ing characters (e.g., color, body dimensions, hind foot
size) supports Cothran’s (1982, 1983) findings of al-
lozymic F { s, F 2 s, and backcross individuals, and genetic
introgression practically guarantees that our subjective
determination of hybrid individuals is conservative.

Ictidomys occupying natural settings tend to be
elusive and sporadically distributed, but the two spe¬

cies are conspicuous and often abundant and readily
collected where they exploit modified circumstances
(e.g., cemeteries, parks, golf courses, school campuses).
Such behavior certainly facilitates observations and
collections for future studies, to include: 1) monitor¬
ing of the conservation status of 7. tridecemlineatus\
2) determining both the full geographic extent of
hybridization between the two species; 3) documenta¬
tion of any intergradation between eastern and western
components of 7 tridecemlineatus\ 4) accuracy of our
methods for hybrid determination; and 5) determina¬
tion of genetic distances between populations at both
specific and subspecific levels as objective measures
of introgression or chronological sequences of events
(e.g., times of origin, divergence, or convergence).
Some of these investigations will require sophisticated
and more sensitive genetic tools, but such preliminary
morphological assessments as ours can provide both
testable hypotheses and avenues for future study.

Acknowledgments

We thank Janet Braun and Marcy Revelez of the
University of Oklahoma’s Stovall Museum of Natural
History for permission to examine Ictidomys speci¬
mens collected by E. Gus Cothran from his study area.
Robert Bradley and Cody Thompson provided useful

discussions and helpful comments on an earlier draft
of the manuscript, and two anonymous reviewers also
contributed materially to the final product. Midwestern
State University provided funding for this study through
faculty research grants to Stangl.

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Addresses of authors:

Frederick B. Stangl, Jr.

Midwestern State University
Wichita Falls, Texas 76308
frederick. stangl@mwsu. edu

Amanda L. Snook

Midwestern State University
Wichita Falls, Texas 76308
amanda_snook@yahoo. com

Mountains, El Paso and Hudspeth counties, Texas.
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Laura A. Harmon

Midwestern State University
Wichita Falls, Texas 76308

Dana R. Mills

Midwestern State University
Wichita Falls, Texas 76308
dana.mills@mwsu. edu

Editor for this manuscript was Robert D. Bradley.

16

Occasional Papers, Museum of Texas Tech University

Appendix

Following is a list of north-central Texas localities and Midwestern State University (MWSU) catalog numbers
for 149 specimens of Ictidomys collected during a 31-county survey from 2004 to 2007, and three laboratory¬
generated F, animals. Soft tissues (liver, heart, kidney, muscle) of each specimen are deposited with the Museum
of Texas Tech University.

Ictidomysparvidens (n = 86).—Baylor Co.: Seymour Masonic Cemetery, 1 (MWSU 22327). Cottle Co.:
Paducah Cemetery, 6 (MWSU 22336-22339,22464,22465). Dickens Co.: Dickens Cemetery, 2 (MWSU 22458,
22463); Spur Cemetery, 4 (MWSU 22383-22386). Fisher Co.: Rotan Cemetery, 2 (MWSU 22340,22341). Garza
Co.: Post Cemetery, 6 (MWSU 22419, 22420, 22422-22424, 22426). Haskell Co.: Haskell, 3 (MWSU 22282-
22284). Jones Co.: Stamford Cemetery, 3 (MWSU 22346-22348); Hamilton Cemetery, 4 (MWSU 22342-22345);
Anson, 1 (22349); 1 mi E of Anson, 10 (MWSU 22350-22354,22363-22367). Kent Co.: Clairemont Fairground,
3 (MWSU 22421, 22427, 22428). King Co.: Guthrie High School campus, 2 (MWSU 22456, 22457); 11.8 mi.
N, 18.3 mi. W Benjamin, 1 (MWSU 22571). Knox Co.: Goree Cemetery, 3 (MWSU 22370-22372); Benjamin
Cemetery, 4 (MWSU 22390,22434-22436); Truscott Cemetery, 1 (MWSU 22560); 4.0 mi. W Truscott, 1 (MWSU
22570). Motley Co.: Matador Cemetery, 6 (MWSU 22381,22382,22459,22460,22466,22467). Stonewall Co.:
Aspermont Cemetery, 4 (MWSU 22406-22409); City of Aspermont, 2 (MWSU 22410,22411). Shackelford Co.:
Albany, 3 (MWSU 22374-22376); Albany Cemetery, 4 (MWSU 22373, 22403, 22404, 22415). Throckmorton
Co.: Throckmorton Cemetery, 10 (MWSU 22261, 22262, 22285, 22291, 22355-22359, 22793).

Ictidomys tridecemlineatus (n = 42).—Archer Co.: Windthorst, 4 (MWSU 22437-22439,22443); Megargel
Cemetery, 2 (MWSU 22441,22442). Crosby Co.: Ralls Cemetery, 3 (MWSU 22425,22432, 22433). Floyd Co.:
Floydada Cemetery, 3 (MWSU 22429-22431). Hardeman Co.: Quanah Cemetery, 4 (MWSU 22308-22311).
Wichita Co.: Kiwanis Park, Wichita Falls, 6 (MWSU 22312-22314, 22316, 22319, 22516); Electra Cemetery,
7 (MWSU 22301-22307); River Creek Golf Course, 10 mi SE Burkburnett, 4 (MWSU 22321, 22322, 22323,
22324); Burkburnett Cemetery, 4 (MWSU 22317-22320); Memorial Stadium, Wichita Falls, 2 (MWSU 22325,
22326). Wilbarger Co.: 0.4 mi NE Lockett, 3 (MWSU 22790, 22794, 22795).

Purported hybrids (n = 21).—Baylor Co.: Seymour Masonic Cemetery, 11 (MWSU 22328-22335, 22413,
22414); Seymour Catholic Cemetery, 1 (MWSU 22440); Old Seymour Cemetery, 1 (MWSU 22412); Salt Creek
Golf Course, Seymour, 2 (MWSU 22788, 22789). Haskell Co.: Haskell Cemetery, 1 (MWSU 22284). Knox
Co.: Goree Cemetery, 2 (MWSU 22371,22372). Throckmorton Co.: Throckmorton Cemetery, 3 (MWSU22286,
22287, 22360).

Laboratory-bred F, animals (n = 3).—Off-spring of female I. parvidens from Albany (Shackleford Co.) and
male I. tridecemlineatus from Wichita Falls (Wichita Co.) (MWSU 22564-22566).

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